Silicon nitride sintered body and silicon nitride heat dissipation substrate

Abstract

To provide a silicon nitride sintered body capable of being manufactured with a manufacturing method having smaller industrial load, even with a silicon nitride sintered body with less auxiliary component, and capable of simultaneously achieving high thermal conductivity and high intensity and a silicon nitride heat dissipation substrate using the same.SOLUTION: A silicon nitride sintered body primarily includes a silicon nitride grain and a grain boundary phase surrounding the silicon nitride grain. The silicon nitride sintered body at least includes a rare-earth element of 1.0 to 6.5 wt.% in total, and an alkaline / alkaline-earth metal element of 0.3 to 3.0 wt.% in total. The grain boundary phase includes an oxynitride-based crystal phase including silicon, and a crystal of a hexagonal boron nitride. In a polished surface of the silicon nitride sintered body, 15% or less of a circumference of a crystal grain of the hexagonal boron nitride are pores.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride sintered body and a silicon nitride heat dissipation substrate. [Background technology]

[0002] Silicon nitride, with its high thermal conductivity and strength, is attracting attention as an insulating heat dissipation substrate for inverter power modules installed in EVs (electric vehicles) and HVs (hybrid vehicles). Traditionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material, but in the case of high-current power modules such as those used in EVs, temperatures reach around 250°C, and the difference in thermal expansion between the substrate and the copper or other metals to which it is bonded generates significant thermal stress, causing the aluminum nitride, which has low strength, to crack or break. For this reason, silicon nitride, which has higher thermal conductivity than common insulating ceramics and even greater strength, is increasingly being adopted, even though its thermal conductivity is inferior to that of aluminum nitride.

[0003] Patent Document 1 discloses a composite ceramic material comprising silicon nitride particles and boron nitride particles, wherein, in a cross section of the composite ceramic material, the silicon nitride particles have an average particle size of 0.1 μm or more and 10 μm or less, and the boron nitride particles have an average particle size of 0.1 μm or more and 10 μm or less, in the cross section, the boron nitride particles have an area ratio of 1% or more and 20% or less of all the silicon nitride particles and boron nitride particles in the cross section, and the composite ceramic material has a porosity of 5% or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-140101 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, as power modules have become hotter, silicon nitride substrates are required to have higher thermal conductivity and thus improved heat dissipation. While reducing the amount of additives and grain boundary phases is effective in improving the substrate's heat dissipation, this results in increased sintering resistance, which increases the sintering temperature and necessitates pressurized sintering, placing a heavy burden on the industrial environment. While adding highly thermally conductive components such as boron nitride to improve thermal conductivity has been considered, these highly thermally conductive components have strong covalent bonds that make atomic diffusion difficult during sintering, resulting in the formation of pores and reduced strength.

[0006] For these reasons, when using silicon nitride sintered bodies as insulating heat dissipation substrates for power devices, there has been a need for a technology that can produce silicon nitride sintered bodies with low additive content using a manufacturing method that imposes a low industrial load and that can achieve both high thermal conductivity and high strength.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a silicon nitride sintered body that can be produced using a manufacturing method that imposes a small industrial load, even if it contains only a small amount of auxiliary components, and that can achieve both high thermal conductivity and high strength, and a silicon nitride heat dissipation substrate using the same. [Means for solving the problem]

[0008] (1) In order to achieve the above object, the silicon nitride sintered body of the present invention employs the following measures: Specifically, the silicon nitride sintered body of an application example of the present invention is a silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles, containing at least 1.0 to 6.5 wt% in total of rare earth elements and 0.3 to 3.0 wt% in total of alkali and alkaline earth metal elements, the grain boundary phase including a silicon-containing oxynitride crystalline phase and hexagonal boron nitride crystals, and on the polished surface of the silicon nitride sintered body, pores cover 15% or less of the periphery of the hexagonal boron nitride crystal particles.

[0009] (2) In the silicon nitride sintered body according to the application example of (1) above, the area ratio of the hexagonal boron nitride crystal grains to the entire area on the polished surface of the silicon nitride sintered body is 3 vol % or less.

[0010] (3) In the silicon nitride sintered body according to the application example of (1) or (2) above, the silicon-containing oxynitride crystal phase contained in the grain boundary phase includes at least one of a melilite phase and a J phase.

[0011] (4) In the silicon nitride sintered body according to any one of the application examples (1) to (3) above, the rare earth element includes one or more elements selected from Y, La, and Yb, and the alkali / alkaline earth metal element includes one or more elements selected from Mg and Ca.

[0012] (5) Furthermore, the silicon nitride sintered body according to any one of the application examples (1) to (4) above further contains a Group 4 element, and the Group 4 element forms one or more compounds at least partly containing nitrogen or carbon.

[0013] (6) A silicon nitride heat dissipation substrate according to an application example of the present invention comprises the silicon nitride sintered body according to any one of (1) to (5) above.

[0014] (7) In addition, in the silicon nitride heat dissipation substrate of the application example of (6) above, the bending strength is 700 MPa or more.

[0015] (8) In addition, in the silicon nitride heat dissipation substrate according to the application example of (6) or (7) above, the thermal conductivity is 80 W / mK or more.

[0016] (9) In the silicon nitride heat dissipation substrate according to any one of the application examples (6) to (8) above, the thickness of the silicon nitride heat dissipation substrate in the direction perpendicular to one of the main surfaces is 220 μm or more and 690 μm or less. [Effects of the Invention]

[0017] The silicon nitride sintered body or silicon nitride heat dissipation substrate of the present invention can provide a silicon nitride sintered body or silicon nitride heat dissipation substrate that combines high thermal conductivity with high strength, and can be produced by a method that imposes little burden on the industrial industry. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic perspective view showing an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view showing an example of a power device using a silicon nitride heat dissipation substrate according to an embodiment of the present invention. [Figure 3] 1 is a table showing the element contents, grain boundary phase characteristics, and properties of each sample. DETAILED DESCRIPTION OF THE INVENTION

[0019] Next, an embodiment of the present invention will be described with reference to the drawings. To facilitate understanding of the description, the same reference numerals are used to designate the same components in the drawings, and duplicated descriptions will be omitted. Note that in the configuration diagrams, the size of each component is shown conceptually and does not necessarily represent the actual dimensional ratio.

[0020] [Embodiment] [Structure of silicon nitride sintered body] First, a silicon nitride sintered body according to an embodiment of the present invention will be described. The silicon nitride sintered body according to an embodiment of the present invention is a silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles. The silicon nitride particles refer to silicon nitride particles or sialon. However, since sialon has a lower thermal conductivity than silicon nitride, it is preferable to set the upper limit of the amount of sialon produced within the range of the Al content described below. "Mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles" means that the body may contain up to 0.5 wt% of impurities other than the elements described below.

[0021] The silicon nitride sintered body contains a total of 1.0 to 6.5 wt% of rare earth elements. If the rare earth element content is less than this range, the sinterability will decrease, pores will remain, and the strength of the silicon nitride sintered body may decrease. If the rare earth element content is more than this range, the amount of grain boundary phase will increase, and the thermal conductivity of the silicon nitride sintered body may decrease. It is believed that the same effect can be obtained regardless of the rare earth element contained. The rare earth element preferably includes one or more elements selected from yttrium (Y), lanthanum (La), and ytterbium (Yb).

[0022] The silicon nitride sintered body contains a total of 0.3 to 3.0 wt% of alkali and alkaline earth metal elements. If the alkali and alkaline earth metal elements are present in amounts less than this range, the strength of the silicon nitride sintered body may be reduced. If the alkali and alkaline earth metal elements are present in amounts greater than this range, the thermal conductivity of the silicon nitride sintered body may be reduced. The alkali and alkaline earth metal elements refer to alkali metal elements or alkaline earth metal elements. The alkali and alkaline earth metal elements preferably include one or more elements selected from Mg and Ca.

[0023] The grain boundary phase of the silicon nitride sintered body contains a silicon-containing oxynitride crystalline phase. Furthermore, the silicon-containing oxynitride crystalline phase preferably contains at least one of the melilite phase and the J phase. The melilite phase is identified by the ICDD (International Centre for Diffraction Data) PDF (Powder Diffraction File) card number 00-045-0249. The melilite phase, also known as the M phase, is a Y2Si3O3N4-type crystal. The J phase is identified by the PDF card number 01-086-1106. The J phase is a Y4Si2O7N2-type crystal. While these crystals show typical crystal compositions, the elements may differ as long as the crystal system, space group, and atomic arrangement are the same. For example, the M phase may be Yb2Si3O3N4.

[0024] The silicon nitride sintered body contains hexagonal boron nitride crystals. Hexagonal boron nitride has high thermal conductivity, so including hexagonal boron nitride crystals in the silicon nitride sintered body can increase thermal conductivity. Furthermore, on the polished surface of the silicon nitride sintered body, pores account for 15% or less of the area surrounding the hexagonal boron nitride crystal grains. By reducing the pores surrounding the hexagonal boron nitride crystal grains, the strength and thermal conductivity of the silicon nitride sintered body can be increased. While it is desirable to have as few pores as possible, boron nitride has strong covalent bonds, making atomic diffusion during sintering difficult, making it difficult to reduce the pores surrounding the hexagonal boron nitride crystal grains to 0%. Therefore, the lower limit of pores may be greater than 0%.

[0025] The proportion of pores surrounding hexagonal boron nitride crystal particles can be determined by SEM (Scanning Electron Microscope) observation. Specifically, five locations are randomly selected on the polished surface of a silicon nitride sintered body, and a 120 μm × 90 μm field of view is observed at 2000x magnification. Next, the contours of all crystal particles recognized as hexagonal boron nitride in the five fields of view are defined. Crystal particles cut off at the edges of the image are not considered. The lengths of the contour lines of the particles recognized as hexagonal boron nitride and the contour lines tangent to the pores in the five fields of view are then summed, and the ratio of the sum of the contour lines tangent to the pores to the total contour lines of the particles recognized as hexagonal boron nitride is calculated to determine the proportion of pores surrounding the hexagonal boron nitride crystal particles. Image analysis software such as Winroof may be used to define the contour lines, calculate the total contour length, and calculate this ratio.

[0026] On the polished surface of the silicon nitride sintered body, the area ratio of hexagonal boron nitride crystal grains to the total area is preferably 3 vol% or less. While the inclusion of hexagonal boron nitride crystals in the silicon nitride sintered body increases the thermal conductivity, too much hexagonal boron nitride content makes the firing conditions more stringent. The lower limit of the area ratio may be, for example, greater than 0 vol%.

[0027] The area ratio of hexagonal boron nitride particles to the total area on the polished surface of a silicon nitride sintered body can be determined by SEM observation. Specifically, five locations are randomly selected on the polished surface of the silicon nitride sintered body, and a 120 μm × 90 μm field of view is observed at 2000x magnification. The areas of the crystal particles recognized as hexagonal boron nitride in the five fields of view are then added together to calculate the ratio to the total area of the field of view, thereby determining the area ratio of hexagonal boron nitride crystal particles to the total area. Image analysis software may also be used to calculate the area ratio.

[0028] The silicon nitride sintered body preferably contains a Group 4 element. The silicon nitride sintered body preferably contains a total of 0.30 to 3.0 wt% of the Group 4 element. The silicon nitride sintered body preferably contains at least one compound in which at least a portion of the Group 4 element contains at least one of nitrogen and carbon.

[0029] When the Group 4 element content is within the above range, both high thermal conductivity and high strength are achieved. Group 4 elements do not dissolve in silicon nitride, preventing a decrease in thermal conductivity due to phonon scattering. Furthermore, they do not dissolve in the grain boundary phase formed by the reaction of rare earth elements with silicon nitride. Therefore, they easily form compounds by themselves, which fill residual porosity and improve strength. Furthermore, the Group 4 element compound particles suppress crack propagation, contributing to high toughness. Note that Group 4 element compounds may include not only compounds containing only Group 4 elements and nitrogen or carbon, but also compounds containing Group 4 elements, nitrogen, and carbon. They may also contain other Group 4 elements or anions (such as oxygen) in solid solution to the extent that the crystal structure is not disrupted. The type of Group 4 element compound can be identified using X-ray diffraction (XRD) results.

[0030] The Group 4 element preferably contains one or more elements selected from zirconium (Zr) and hafnium (Hf). This allows for the specific formation of a Group 4 element compound. When the silicon nitride sintered body contains Zr, the Group 4 element compounds that are likely to be formed are zirconium nitride (ZrN), zirconium carbide (ZrC), and zirconium carbonitride (ZrCN). When the silicon nitride sintered body contains Hf, the Group 4 element compounds that are likely to be formed are hafnium nitride (HfN), hafnium carbide (HfC), and hafnium carbonitride (HfCN).

[0031] The silicon nitride sintered body may contain more than 0 wt% but not more than 0.1 wt% Al. Al is an impurity derived from the raw materials, and its inclusion improves the sinterability of the silicon nitride sintered body, thereby increasing its strength. Furthermore, as long as the content is within this range, the effect of the decrease in thermal conductivity due to the formation of sialon is virtually negligible. If the content is greater than 0.1 wt%, the amount of sialon in the silicon nitride sintered body increases, which may result in a decrease in the thermal conductivity of the silicon nitride sintered body.

[0032] These features enable the production of silicon nitride sintered bodies that combine high thermal conductivity with high strength, and can be produced using a manufacturing method that places minimal burden on the industrial sector.

[0033] [Configuration of silicon nitride heat dissipation substrate] FIG. 1 is a schematic perspective view showing an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention. The silicon nitride heat dissipation substrate 10 of the present invention is made of the silicon nitride sintered body described above. This allows for both high thermal conductivity and high strength, reducing the risk of defects occurring in circuit boards using the same. The silicon nitride heat dissipation substrate 10 of the present invention can be suitably used as a heat dissipation substrate for circuit boards for power devices. The silicon nitride heat dissipation substrate 10 is formed, for example, in a flat plate shape.

[0034] The silicon nitride heat dissipation substrate 10 preferably has a bending strength of 700 MPa or more, which reduces the risk of the silicon nitride heat dissipation substrate 10 being damaged.

[0035] Bending strength can be measured as follows. In accordance with ISO 23242, a silicon nitride heat dissipation substrate is processed to a fixed thickness of 12 mm x 25 mm. Then, bending strength can be measured using a 15 mm span and three-point bending test. ISO 23242 is applicable to ceramic thin plates with a thickness of 0.2 mm to 1.0 mm.

[0036] The silicon nitride heat dissipation substrate 10 preferably has a thermal conductivity of 80 W / mK or more, which allows it to fully exhibit its performance as a heat dissipation substrate.

[0037] The thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed to a size of 0.32 mm x 10 mm, and the thermal diffusivity is measured by the xenon flash method. The density of the silicon nitride heat dissipation substrate 10 is also measured by a method in accordance with JIS R1634. The specific heat value is 0.68 cm 2 The thermal conductivity can be calculated from the measured thermal diffusivity and density values using the formula (thermal conductivity) = (density) x (specific heat) x (thermal diffusivity).

[0038] The thickness of the silicon nitride heat dissipation substrate 10 in the direction perpendicular to one of its main surfaces is preferably 220 μm or more and 690 μm or less. This allows for a good balance between the strength and heat dissipation of the silicon nitride heat dissipation substrate 10. If the thickness is smaller than this range, the strength of the substrate may decrease. If the thickness is larger than this range, the heat dissipation performance may decrease.

[0039] [Power device configuration] 2 is a schematic cross-sectional view showing an example of a power device using a silicon nitride heat dissipation substrate according to an embodiment of the present invention. The power device 100 includes a circuit board 20, a power semiconductor 30, a heat sink 40, and a heat dissipation member 50.

[0040] The circuit board 20 comprises a silicon nitride heat dissipation substrate 10 having a circuit layer 12 formed on one main surface thereof and a conductor layer 14 formed on the other main surface thereof opposite the one main surface. The circuit layer 12 and the conductor layer 14 are preferably made of metal, and more preferably made of a metal containing copper as a main component. The circuit layer 12 and the conductor layer 14 are bonded to the silicon nitride heat dissipation substrate 10 directly or with a bonding material such as brazing material.

[0041] A power semiconductor 30 is mounted on the upper side of the circuit layer 12 of the circuit board 20. The power semiconductor 30 and the circuit layer 12 may be joined using solder 22 or the like. The power semiconductor 30 may be, for example, a semiconductor for use in an EV that carries a large current and is prone to high temperatures. The silicon nitride heat dissipation substrate 10 of the present invention has high strength while maintaining high thermal conductivity, and is therefore less likely to crack or break even if high temperatures cause large thermal stress in the silicon nitride heat dissipation substrate 10 due to the difference in thermal expansion between the silicon nitride heat dissipation substrate 10 and the metal to which it is joined.

[0042] A heat sink 40 is bonded to the underside of the conductor layer 14 of the circuit board 20. The heat sink 40 and the conductor layer 14 may be bonded using solder 22 or the like. The surface of the heat sink 40 opposite the surface bonded to the conductor layer 14 is in contact with a heat dissipation member 50 via grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal containing copper as its main component. The heat dissipation member 50 has heat dissipation fins formed thereon. The heat dissipation member 50 is preferably made of metal, and more preferably made of a metal containing copper or aluminum as its main component.

[0043] [Methods for producing silicon nitride sintered bodies and silicon nitride heat dissipation substrates] An example of a method for manufacturing the silicon nitride sintered body and silicon nitride heat dissipation substrate is shown below. First, the necessary raw material powders for the silicon nitride sintered body are selected and weighed to achieve the desired composition. The raw material powder for the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of the elements contained in the silicon nitride sintered body. In addition to silicon nitride (Si3N4), examples of raw material powders for the silicon nitride sintered body include magnesium carbonate (MgCO3), calcium carbonate (CaCO3), yttrium oxide (YO3), ytterbium oxide (Yb2O3), lanthanum oxide (La2O3), and zirconium nitride (ZrN).

[0044] Ethanol is added to these raw material powders, and the mixture is wet mixed and pulverized in a ball mill for, for example, 6 to 60 hours to obtain a slurry. The slurry is dried in a hot water bath or spray dryer to obtain a mixed powder. A binder (acrylic or the like) may be added to the obtained mixed slurry, and the mixture may be mixed for about 30 minutes and then dried.

[0045] Next, the mixed powder is filled into a mold and uniaxially pressed at a pressure of, for example, 100 MPa to form the desired shape, obtaining a green body. The green body may be formed by cold isostatic pressing (CIP). Next, hexagonal boron nitride (hBN) is applied to the surface of the green body. For example, hBN powder can be dissolved in ethanol and applied to the surface of the green body. The hBN used here is subjected to a surface oxidation treatment in advance. For example, the surface oxidation treatment can be performed by heating the hBN powder in an air atmosphere at approximately 1000°C, or by adding it to water and mixing it for approximately 24 hours. Next, the obtained green body is subjected to a heat treatment (debinding) by, for example, holding it in a nitrogen flow at a temperature of 600°C for 5 hours, obtaining a debound body.

[0046] The resulting degreased body is then placed in, for example, a silicon carbide mold with the inside coated with BN, and sintered, for example, at a maximum temperature of 1900°C for 5 hours in a nitrogen atmosphere at 9 atmospheres. The cooling rate to 1400°C is adjusted to, for example, about 100°C / hr. A silicon nitride sintered body is obtained through this process.

[0047] When the silicon nitride sintered body is used as a silicon nitride heat dissipation substrate, the outer shape is processed to a predetermined shape and thickness by, for example, grinding, polishing, blasting, etc.

[0048] By using this manufacturing method, it is possible to manufacture a silicon nitride sintered body or a silicon nitride heat dissipation substrate that has both high thermal conductivity and high strength.

[0049] [Examples and Comparative Examples] (Sample 1) 96.5 wt% silicon nitride powder (average particle size 1.4 μm), 2.0 wt% yttrium oxide powder (average particle size 1.0 μm), and 1.5 wt% magnesium carbonate powder (average particle size 2.5 μm) were weighed out. Next, the weighed raw material powders were ball milled to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were placed in a resin pot and milled and mixed at 60 rpm for 24 hours using YSZ (Y2O3 partially stabilized zirconia) balls. A binder (acrylic) was added to the obtained mixed slurry, and after mixing for an additional 30 minutes, the slurry was dried by spray drying to obtain a mixed powder.

[0050] Next, the resulting mixed powder was subjected to powder press molding using a uniaxial press to produce a compact. After filling a dedicated mold with the mixed powder, it was compacted using a uniaxial press at a pressure of 100 MPa. Next, hexagonal boron nitride powder (hBN powder, average particle size 7.0 μm) was dissolved in ethanol and applied to the surface of the compact. The hBN powder used here had previously been subjected to a surface oxidation treatment by heating it in air at 1000°C for 1 hour.

[0051] The resulting green body was then degreased. This was done by holding it in a nitrogen flow at a maximum temperature of 600°C for 5 hours. The resulting degreased body was then fired. The firing method was atmospheric firing under a nitrogen gas pressure of 9 atmospheres, with the maximum temperature being 1900°C held for 5 hours. The cooling rate to 1400°C was 100°C / hr. A silicon carbide mold with a BN coating on the inside was used. In this way, the silicon nitride sintered body of Sample 1 was produced.

[0052] (Sample 2) The silicon nitride sintered body of Sample 2 was produced under the same conditions as those for the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 3.0 wt %.

[0053] (Sample 3) The silicon nitride sintered body of Sample 3 was produced under the same conditions as those for the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 8.0 wt %.

[0054] (Sample 4) The silicon nitride sintered body of Sample 4 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 3.0 wt%, the amount of magnesium carbonate powder added was 1.0 wt%, and 0.5 wt% of calcium carbonate powder (average particle size 2.5 μm) was also added.

[0055] (Sample 5) The silicon nitride sintered body of Sample 5 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 3.0 wt % and the amount of magnesium carbonate powder added was 5.0 wt %.

[0056] (Sample 6) The silicon nitride sintered body of Sample 6 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 μm).

[0057] (Sample 7) The silicon nitride sintered body of Sample 7 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the yttrium oxide powder was changed to lanthanum oxide powder (average particle size 1.0 μm).

[0058] (Sample 8) The silicon nitride sintered body of Sample 8 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the surface oxidation treatment of the hBN powder was changed to a method of mixing in water for 24 hours.

[0059] (Sample 9) The silicon nitride sintered body of Sample 9 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that carbon powder was placed in a BN-coated silicon carbide mold, and then the degreased body was placed in the mold and fired.

[0060] (Sample 10) The silicon nitride sintered body of Sample 10 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the balls used to produce the mixed slurry were changed from YSZ to silicon nitride.

[0061] (Sample 11) The silicon nitride sintered body of Sample 11 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that hBN powder that had not been subjected to surface oxidation treatment was used and the degreasing process was carried out in air flow.

[0062] (Sample 12) The silicon nitride sintered body of Sample 12 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that 2.0 wt% of hBN powder that had not been subjected to surface oxidation treatment was added when the mixed powder was produced, and hBN was not applied to the surface of the compact.

[0063] (Sample 13) The silicon nitride sintered body of Sample 13 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that 3.0 wt% of hBN powder that had not been subjected to surface oxidation treatment was added when the mixed powder was produced, and hBN was not applied to the surface of the compact.

[0064] [Various measurements] The sintered body of each sample was polished to remove 0.1 mm from the surface, and then evaluated by the following measurements.

[0065] (density measurement) The density of the sintered body of each sample was measured by a method in accordance with JIS R1634.

[0066] (Measurement of elemental amounts) The polished surfaces were subjected to X-ray fluorescence analysis (XRF) to measure the types and amounts of constituent elements of the sintered body of each sample.

[0067] (Identification of constituent phases) The crystalline phase of the sintered grain boundary phase of each sample was identified by XRD analysis. The hBN crystal grains were also identified by SEM-EDS analysis. However, the table in Figure 3 lists only the components identified as crystalline, and does not include information on whether or not there is amorphous content.

[0068] (Measurement of pores around hBN crystal particles) The hBN crystal particles identified by SEM-EDS analysis were observed using SEM, and the proportion of pores surrounding the hBN crystal particles was calculated using image analysis software.

[0069] (Measurement of the area ratio of hBN crystal grains) The hBN crystal particles identified by SEM-EDS analysis were observed using SEM, and the area ratio of the hBN crystal particles was calculated using image analysis software.

[0070] (Calculation of thermal conductivity) The sintered body of each sample was processed into a 0.32mm x 10mm square, and the thermal diffusivity was measured using the xenon flash method. The specific heat value was 0.68cm 2 The thermal conductivity was calculated from the thermal diffusivity measured by the xenon flash method and the density value above using the formula (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0071] (Bending strength measurement) In accordance with ISO23242, the sintered body of each sample was processed into a thickness of 0.32 × 12 × 25 mm, and the bending strength was measured using a 15 mm span, three-point bending test.

[0072] (result) Figure 3 is a table showing the element content, grain boundary phase characteristics, and properties of each sample. The Group 4 elements in each sample except for sample 10 originate from the YSZ balls used in the manufacturing process. Samples 1 to 10 all had high thermal conductivities of 80 W / mK or more and bending strengths of 700 MPa or more.

[0073] Furthermore, hBN particles were confirmed inside the silicon nitride sintered compacts of all samples 1 to 10. This is thought to be because the surface of the compacts was coated with hBN particles that had been subjected to a surface oxidation treatment, and then degreased and sintered, causing the hBN to diffuse into the interior of the sintered compacts.

[0074] Sample 11 had a low thermal conductivity. This is presumably because no silicon-containing oxynitride crystal phase was formed in the grain boundary phase. The reason why no silicon-containing oxynitride crystal phase was formed is that the hBN powder applied to the compact surface was not subjected to a surface oxidation treatment. The degreasing process was carried out in an air atmosphere to induce oxidation, but the oxides produced during this process are thought to have inhibited the formation of a silicon-containing oxynitride crystal phase in the grain boundary phase.

[0075] Sample 12 had low thermal conductivity, bending strength, and bulk density. This is presumably because the addition of hBN powder without surface oxidation treatment to the mixed powder resulted in poor sintering of the silicon nitride sintered body, resulting in the formation of pores exceeding 15% around the hBN crystal particles.

[0076] Sample 13 had low thermal conductivity and bending strength, as well as low bulk density. This is presumably because the addition of more non-surface-oxidized hBN powder to the mixed powder than in Sample 12 resulted in poorer sinterability of the silicon nitride sintered compact, resulting in the formation of pores in excess of 15% around the hBN crystal particles, and the proportion of hBN hexagonal boron nitride particles to the total area on the polished surface of the silicon nitride sintered compact exceeding 3 vol%.

[0077] To obtain sintered bodies with sufficiently high thermal conductivity and bending strength using the method of adding hBN particles, as in Samples 12 and 13, it is thought that it is necessary to use an industrially demanding sintering method, such as extending the sintering time, increasing the sintering temperature, or sintering under pressure using a press, etc. Therefore, when adding hBN to silicon nitride sintered bodies, it was found that it is preferable to apply hBN that has been surface-oxidized to the surface of the compact before sintering.

[0078] A comparison of Sample 2 and Sample 9 revealed that the Group 4 element compounds contained in the silicon nitride sintered body play equivalent roles to compounds containing a Group 4 element and nitrogen and compounds containing a Group 4 element, nitrogen, and carbon. It is also presumed that the Group 4 element compound contained in the silicon nitride sintered body may be a compound containing a Group 4 element and carbon.

[0079] Sample 10 had slightly lower thermal conductivity and bending strength. This is presumably because the silicon nitride sintered body did not contain a Group 4 element compound. This confirmed that it is preferable for silicon nitride sintered bodies to contain a Group 4 element compound. Meanwhile, Sample 10 had higher thermal conductivity and bending strength than Samples 12 and 13. This means that even silicon nitride sintered bodies that do not contain a Group 4 element compound, as long as they contain hexagonal boron nitride and satisfy the criteria that 15% or less of the periphery of the hexagonal boron nitride crystal grains are pores, can have higher thermal conductivity and bending strength than those that do not satisfy the criteria.

[0080] From the above results, it has been confirmed that the silicon nitride sintered body and silicon nitride heat dissipation substrate of the present invention can be a silicon nitride sintered body or a silicon nitride heat dissipation substrate that combines high thermal conductivity and high strength, and can be produced by an industrially low-burden method.

[0081] The present invention is not limited to the above-described embodiments, and various modifications and equivalents are included within the spirit and scope of the present invention. Furthermore, the structure, shape, number, position, size, etc. of the components shown in each drawing are for the convenience of explanation and may be changed as appropriate. [Explanation of symbols]

[0082] 10 Silicon nitride heat dissipation substrate 11 Main surface 12 circuit layers 14 Conductor layer 20 Circuit Board 22 Solder 30 Power Semiconductors 40 Heat sink 42 Grease 50 Heat dissipation material 100 Power Devices

Claims

1. A silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles, Contains at least 1.0 to 6.5 wt% of rare earth elements in total and 0.3 to 3.0 wt% of alkali / alkaline earth metal elements in total, the grain boundary phase includes a silicon-containing oxynitride-based crystalline phase, A silicon nitride sintered body comprising hexagonal boron nitride crystals, wherein pores cover 15% or less of the periphery of each hexagonal boron nitride crystal grain on a polished surface of the silicon nitride sintered body.

2. 2. The silicon nitride sintered body according to claim 1, wherein the area ratio of the hexagonal boron nitride crystal grains to the entire area of the polished surface of the silicon nitride sintered body is 3 vol % or less.

3. 3. The silicon nitride sintered body according to claim 1, wherein the silicon-containing oxynitride crystalline phase contained in said grain boundary phase includes at least one of a melilite phase and a J phase.

4. the rare earth element includes one or more elements selected from Y, La, and Yb, 3. The silicon nitride sintered body according to claim 1, wherein said alkali / alkaline earth metal element includes at least one element selected from the group consisting of Mg and Ca.

5. Further containing a Group 4 element, 3. The silicon nitride sintered body according to claim 1, wherein said Group 4 element forms one or more compounds at least a portion of which contains nitrogen or carbon.

6. 3. A silicon nitride heat dissipating substrate comprising the silicon nitride sintered body according to claim 1.

7. 7. The silicon nitride heat dissipation substrate according to claim 6, having a bending strength of 700 MPa or more.

8. 7. The silicon nitride heat dissipation substrate according to claim 6, having a thermal conductivity of 80 W / mK or more.

9. 7. The silicon nitride heat dissipation substrate according to claim 6, wherein the thickness of said silicon nitride heat dissipation substrate in a direction perpendicular to one of its main surfaces is 220 [mu]m or more and 690 [mu]m or less.

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